The present invention relates to an ultrasonic imaging system and method which image an image in an object using ultrasonic waves.
An ultrasonic imaging system used in medical imaging diagnosis can display and observe a tomographic image of the tissue of a soft part of a living body and an image of the blood flow flowed in a living body in substantially real time on a monitor using a pulse echo method. The ultrasonic imaging system also gives no radiation exposure of an imaging diagnostic system using radiation to an object, which offers high safety. Further, it is small and inexpensive. It is used widely in a medical field.
The imaging mechanism of a prior art ultrasonic imaging system will be described as follows. The ultrasonic imaging system transmits an ultrasonic wave from an ultrasonic probe having an array of piezoelectric materials to an object. Timing transmitting the ultrasonic wave from the devices of the array is controlled. The focal point position of the ultrasonic wave in the object is controlled. A signal which has been received by each of the devices of the array is added by shifting time according to a distance difference between a desired position and the position of each of the devices to strengthen a signal from the desired position. An ultrasonic pulse is scanned over the entire imaging region to acquire a tomographic image. In the prior art, in such pulse echo method, an echo (receive signal) from an interface in which acoustic impedance in an object is changed is received to envelope detect the receive signal so as to use an absolute value of the echo for imaging a tomographic image.
Prior art about trials to perform imaging while the code of an echo remains or using phase information is proposed (for example, see Japanese Patent Application Laid-Open No. 55-136043 and Japanese Patent Application Laid-Open No. 11-137546).
In the prior art methods, when using code information of an echo (receive signal), phase shift of an echo signal due to a change in acoustic impedance cannot be discriminated from phase shift of an echo signal due to other factors. It is difficult to image an acoustic impedance map of a real object.
The above-described other factors are the following two factors (1) and (2).
(1) Since the center frequency of an ultrasonic pulse is changed by attenuation in a living body, an apparent phase is shifted. (2) Due to phase shift in the lateral direction of an ultrasonic beam, a signal from a noted echoing object near the ultrasonic beam is received as a phase shift signal.
A first object in such prior art methods is to correct a phase shift effect of a receive signal of (1) and (2) to image a real change in acoustic impedance in a living body.
A second object is an object about monitoring the treating effect of a coagulation therapy described below. The coagulation therapy is a method for thermally treating the affected part of prostatomegaly, prostatic cancer or liver cancer by irradiating strong focusing ultrasonic waves or irradiating electromagnetic waves such as RF waves or microwaves. The above-described phase information imaging is expected to accelerate development of a minimally invasive therapy using coagulation which has been developed in recent years.
In these therapies, treatment can be conducted without making a large incision on the surface of a body. Their clinical use has been enlarged, which makes an effect decision method important. When there is a treating leak of cancer, the cancer grows therefrom. It is important to decide whether a part to be treated has been completely treated. At present, contrast CT, contrast MRI, and contrast ultrasonic wave methods are used. It is difficult to perform imaging in the CT or MRI during treatment. Since a contrast agent is used, treatment is stopped once to perform effect decision. When it is failed, the treatment is restarted.
To conduct more effective treatment, it is desired that effect decision can be performed while conducting treatment. Distortion imaging using a pressurizing method has been studied as a tissue imaging method without using an ultrasonic contrast agent. This is considered to be applied only to a location where pressurizing can be performed effectively. From the above reasons, a simplified tissue hardness imaging method is necessary.
A third object is an object about a contrast ultrasonic wave method. The contrast ultrasonic wave method is a method for observing a time difference between the functions of a living body using a behavior injecting a contrast agent as a trigger. This can image the contrast of a difference in blood vessel system for each tissue of a liver (that is, arterial dominant for a liver cancer and portal dominant for other normal tissue parts).
In this case, it is important that a signal from a contrast agent be discriminated from a signal from the tissue of a living body for imaging. In the prior art, a contrast between echo signals from a contrast agent and the tissue of a living body is provided by a method for using a frequency change in an echo signal such as using nonlinearity of the response of a contrast agent to employ a second harmonic wave. Since the tissue of a living body also has nonlinearity, the contrast is hard to provide. Improvement in the contrast is desired.